1. Field of the Invention
The invention relates to the fields of vocoders, transmitting analog signals in digital form and synthesizing analog signals.
2. Prior Art
Digitization of analog signals, particularly voice waveforms has become more emphasized in recent years. No doubt, this interest has been encouraged by the rapid development of digital circuits, the benefits inherent in digital transmission and the expectations of data compression. Moreover, digital voice channels more readily permit secured communications.
The so-called "vocoder" methods provide techniques for analyzing speech patterns which permit transmission, in digital form, of data used to synthesize voice. Vocoder methods generally operate differently upon voiced speech and unvoiced or fricative speech, thus a system must distinguish between these two speech forms and provide alternate means for unvoiced speech.
The vocoder methods for voiced speech determine a pitch component and data representing vocal tract structure known as the "formants." Both pitch extraction and determination of formant data have presented formidable problems, particularly where multiple voices and or interference including periodic noise are present.
In general, the prior art techniques have presented the separate determinations of pitch and formant data as prerequisites to vocoding. See IEEE Spectrum, October 1973, "Voice Signals: Bit-by-bit," pages 28-34; and IEEE Spectrum, August 1970, "Speech Spectograms Using the Fast Fourier Transform," pages 57-62.
The presently disclosed system does not require a determination between voiced and unvoiced speeh. Moreover, the system does not rely upon a separate pitch extraction.